Turbine blade repair

ABSTRACT

A method of repairing a turbine blade which includes performing a heating operation on the blade in which the blade is heated to achieve a predetermined temperature distribution, sensing the temperature of the blade at least at one position by temperature sensing means and controlling the application of heat in accordance with the temperature determined by said at least one sensing means and monitoring the temperature of the blade at a location adjacent said at least one position with a monitoring means.

This invention relates to a method of repairing turbine blades.

In use, turbine blades are subject to at least one of erosion damage andcracking due to high stress in the blade.

The tip of a turbine blade in a rotor assembly is the part of the bladewhich experiences maximum velocity and is hence the part subject to thegreatest erosion.

The present invention relates primarily but not exclusively to bladesused in steam turbines. Erosion or damage of steam turbine bladesgenerally occurs following contact between the turbine blades and eitherparticles carried by the steam or water droplets.

Damage can occur from metal or other solid particles, for example boilerscale, entering a steam turbine with the steam flow and striking theblades. Water droplet damage is generally restricted to the final stagesof a turbine. However, all stages of a turbine can be affected byparticle damage.

The rate of erosion of turbine blades is dependent upon the relativespeeds between the blades and the particles or water droplets causingthe erosion and it is generally the leading edge at the outer end ofeach blade that is most prone to erosion by impact.

At low pressure end of a steam turbine the steam is at a lowertemperature having been cooled during passage through the turbine and atthe dew point water droplets precipitate giving rise to "wetter" steam.The transfer of energy by impact in the turbine blades of the steam istherefore greater at the lower pressure end than at the high pressureend of the turbine. Once again it is predominantly the outer ends of theblades which are especially subject to erosion.

To reduce the rate of erosion, it is usual to provide a shield at theleading edge of the outer end of the turbine blade, such a shield beingmade from a material harder than the blade material.

Steam turbine blades may be made from many suitable materials, a typicalexample being a 12-13% chrome steel and the shield may be made fromstellite or a suitably hardened steel. The shield may be brazed orwelded to the blade.

After a period of use it is often found that the shield has beenconsiderably eroded and one accepted method of repair is to first removethe old shield and secure a new shield in place either by brazing orwelding the new shield to the blade or building up a new shielddepositing fused metal in a welding operation. The blade is thenmachined in the area of the repair to return it to its original aerofoilprofile.

Cracks in turbine blades due to stress in the blade usually occur atstress concentrations and notches found near discontinuities in theblade aerofoil or root section.

Turbine rotors generally speaking have two types of blades:

(a) Free-standing, secured to the rotor only at the root but which haveno connection between adjacent blades;

(b) "Tied" blades which are additionally connected to each other bycover bands connecting each blade by tenons at the end of the bladesecured to a shroud passing round the ends of all the blades, or "lacingwires" which pass through holes in the blades, each blade being brazedor otherwise secured to the lacing wires. Some turbine blades areprovided with snubbers, snubbers being projections from the lower andupper surfaces of the blade to connect with similar projections from theupper and lower surfaces of adjacent blades.

Repair of the two types of turbine blades involving welding or brazingsteps may lead to deformation of the blades. In the case offree-standing blades the blade is allowed to deform during theapplication of heat from the welding or brazing stage and thereafter maybe mechanically returned to its original shape, i.e. by bending. Withblades which are tied to each other by lacing wire, snubbers, etc., theapplication of substantial thermal energy to the blade during a weldingor brazing operation does not deform the blade to such an extent sinceunrestricted movement of the blade is not permitted due to the presenceof the lacing wires, etc. Since the blade is not able to freely moveconsiderable stress is imparted to the blade the problems associatedwith such stress not being apparent until operation of the turbine whencracks may appear in the stressed areas.

It has also been found that even with blades that are freestanding themechanical force needed to return the blade to its original form itselfimparts stress to various areas of the blade which can lead to prematurefailure of the blade.

The problem of premature cracking of the blade after repair occurs withboth types of blades mentioned above and is associated with any type ofrepair to the blade in which substantial thermal energy is applied tothe blade normally during a welding or brazing operation.

For these, and other reasons, repair of a turbine blade may requireperforming a heating operation on the blade in which the blade is heatedto achieve a predetermined temperature distribution.

The heating operation may be performed as a preheating operation beforethe welding operation or during or after the welding operation, forexample as an annealing or stress relief operation.

It is important that the heating operation is performed so that theblade is heated accurately to achieve the predetermined temperaturedistribution and to this end temperature sensing means are provideddisposed at positions so as to sense the temperature of the blade atsaid positions and thereby to control the application of heat so thatthe predetermined temperature distribution is achieved.

However, if the above mentioned temperature sensing means failaccurately to sense the temperature at one or more of said positions theapplication of heat is incorrectly controlled and the predeterminedtemperature distribution is not achieved. Even if incorrect heating isnoticed, for example, as a result of the blade area concerned glowing atdifferent colour to other blade parts, it is necessary to heat treatthat blade after the other blades and this can incur a long time delay,for example of 12 hours.

An object of the invention is to provide a method of repairing a turbineblade in which the above mentioned problem is overcome or is reduced.

According to the present invention we provide a method of repairing aturbine blade which includes performing a heating operation on the bladein which the blade is heated to achieve a predetermined temperaturedistribution, sensing the temperature of the blade at least at oneposition by temperature sensing means and controlling the application ofheat in accordance with the temperature determined by said at least onesensing means and monitoring the temperature of the blade at a locationadjacent said at least one position with a monitoring means.

Preferably the method comprises the steps of applying heating means toselected areas of a turbine blade to be repaired, heating said areas ofthe turbine blade to desired temperatures, sensing the temperature ofthe blade at positions associated with said areas by respectivetemperature sensing means, and controlling the application of heat bysaid heating means in accordance with the temperature determined by therespective temperature sensing means and monitoring the temperature ofthe blade at a location adjacent each of said positions with saidmonitoring means.

The monitoring may be performed with a temperature sensitive deviceseparate from a temperature sensing means by which the temperature atsaid at least one position is sensed.

The monitoring may comprise the manual monitoring of an indicatingmeans. The indicating means may comprise at least one of an audible anda visible alarm signal. The indicating means may be activated as aresult of the temperature sensitive device sensing that the temperatureof the blade at said location lies outside said predeterminedtemperature distribution.

Alternatively or in addition, the monitoring means may include a visualdisplay unit which may indicate the temperature sensed by the monitoringmeans and/or provide a historic record of temperature displayed againsttime.

The monitoring means may provide a control signal to a heating unit todisable or otherwise control the heating unit if the temperature sensedby the monitoring means lies outside predetermined range.

The monitoring means may be substituted for the temperature sensingmeans to control the application of heat, for example, if thetemperature sensing means fails or is inaccurate.

The monitoring means may be disposed in a predetermined relationship tosaid at least one temperature sensing means and the monitoring means maybe responsive to a temperature sensed by the monitoring means which isrelated to the temperature sensed by the associated sensing means in apredetermined manner.

The method of repair may be carried out in situ, for example, withoutremoval of the blade from its rotor or its stator as the case may be.

The method may include the step of carrying out a repair step on saidblade which involves the application of substantial thermal energy tothe turbine blade.

The method may comprise a welding or brazing operation.

The welding operation may comprise the deposition of weld metal torebuild a part of the blade and/or the welding operation may comprisemaking a welded joint to join a repair element to a residual part of theblade.

The method of repair of the present invention is suitable for manyrepairs requiring a welding or brazing step such as welding or brazingcracks in a blade and/or re-building by welding or brazing and/orsecuring by welding or brazing a hard metal shield at the outer end ofthe leading edge of a turbine blade.

The method of repair may comprise the steps of removing metal from theturbine blade in the area of a crack or other defect, replacing theremoved material with metal in a fused state by a welding or brazingoperation and/or by securing an insert of solid metal to the blade by awelding or brazing operation.

The blade may be heated in a controlled manner to a predetermineddesired temperature before and/or during and/or after the repairoperation.

If desired, before, during or after the repair operation, heat may beapplied to the turbine blade and the turbine blade may be maintained ata temperature, or a series of different temperatures, for apredetermined length of time such as to eliminate or substantiallyreduce, stress present in said blade.

The turbine blade may be maintained at a predetermined temperatureduring the welding or brazing operation.

The method may include the step of removing a surface layer from atleast part of the blade and then performing a hardness testing operationto determine a desired temperature distribution.

The blade may have been thermally hardened prior to removing saidsurface layer.

A plurality of blades may be repaired by first achieving a desiredtemperature distribution for a sample blade by determining a heatingspecification by selecting, positioning and energising at least oneheating means on the sample blade, for example empirically, and thenapplying a similar heating specification to each other blade.

The determination of the heating specification of the sample blade maybe performed whilst heating the sample blade to a temperature below thatin which a metallurgical effect occurs on the blade.

The sample blade may be provided with a plurality of monitoring means,at spaced positions thereon, associated with a single temperaturesensing means and associated heating means, whilst each other blade maybe provided with a lesser number of, preferably only one, monitoringmeans associated with a single temperature sensing means and associatedheating means.

Conveniently, electrical heating means are provided to heat the turbineblade to the required temperature.

Control of the temperature of the turbine blade by the use of electricalheating means enables satisfactory control of the desired temperaturesto be achieved.

The invention will now be described in more detail by way of examplewith reference to the accompanying drawings, wherein:

FIG. 1 is a front elevation of a turbine rotor assembly,

FIG. 2 is a view of an end region of a turbine blade of the assembly ofFIG. 1,

FIG. 3 is a view of an intermediate part of a turbine blade of theassembly of FIG. 1,

FIG. 4 is a diagrammatic illustration of the heating means for theturbine blade of FIG. 2, and

FIG. 5 is a similar view to that of FIG. 4 but in respect of the turbineblade shown in FIG. 3.

FIG. 1 illustrates an example of a rotor from a turbine and comprises aplurality of turbine blades 10 having a root part 11 by which theturbine blades are attached to a rotor 12. The blades may beinterconnected by lacing wires and at their outer ends may be connectedto a shroud by tenons in conventional manner.

The turbine blades may be made from a variety of material depending uponthe functional operating conditions of the turbine. A typical materialfrom which a blade in a steam turbine may be manufactured is chromesteel with approximately 12-13% chromium by weight.

Referring now to FIG. 2 the area of the turbine blade most prone toerosion is illustrated as being the leading edge 13 of the outer end ofthe turbine blade. It is this part of the blade which collides withsteam or particles carried thereby during use of the turbine and whichalso has the greatest peripheral velocity being the radially outermostpoint from the axis of rotation. The area where erosion is most likelyto occur is illustrated at 14 in FIG. 2.

Such a turbine blade can be repaired in a number of different waysdepending upon the particular circumstances. The modes of repair includeremoving material, for example by grinding, from the turbine blade inthe region 14, the amount of material being removed ensuring that allthe damaged material is removed. A new hard shield is then built up bydeposition of fused metal in a brazing or a welding operation. The hardmaterial may, for example, be stellite. Alternatively, material may beremoved by grinding or cutting from the turbine blade bordering theleading edge 13, again removing all the damaged material, and thenwelding in place an insert of suitable solid material. For example, acomposite insert having a leading edge shielded with hard material suchas stellite and a trailing part of material compatible with the parentmaterial of the blade and welded thereto.

Referring now to FIG. 3 a turbine blade is shown at 110 in which a crack111 has occurred. Such a blade is repaired by removing material from thecrack as shown by the chain dotted line 112 to form a slot. After theslot has been formed it is closed by deposition of weld material.Thereafter the blade is machined to remove excess weld material andprovide a blade of the desired shape.

In all of the repair methods described hereinbefore, and in other repairmethods which may be used, it is necessary to heat the blade. The blademay be preheated, and/or during and/or after the performance of awelding or brazing operation.

The temperature to which different parts of the blade are heated willdepend upon the material from which the blade is made, the dimensions ofthe blade and the type of repair to be carried out.

The repair method illustrated in FIG. 2 may involve pre-heating theblade, for example, to a temperature of about 400° C. in the repairarea, whilst different parts of the blade may be heated to differenttemperatures which may be higher or lower. The repair method illustratedin FIG. 3 may be performed with a pre-heating step of raising the bladeto, for example, approximately 200° C.

Where appropriate, the blade may be heated before, during or aftercompletion of the welding operation, for example to a temperature lyingin the range of 700° C. to 800° C. for a predetermined length of time torelieve the blade of any residual stress.

Material used for the welding process may be of any suitable materialcompatible with material from which the blade is made, which asmentioned above, may be 12%-13% chromium steel. Alternatively thewelding material may comprise an alloy capable of bonding satisfactorilyto the parent metal of the blade by which welding material is inherentlymore resilient than that of the blade. A suitable welding material isInconel which comprises approximately two thirds nickel, one thirdchrome plus other metallic elements in small quantities.

When a set of blades are presented for repair (they may be in situ, forexample, mounted on the rotor or a stator) it may first be ascertainedwhether the blades have been thermally hardened, for example by flamehardening or induction hardening.

For example, if the blade is made of relatively low carbon steel, e.g.12% Cr, 1% C, such a blade would not be thermally hardened. However, ifthe blade has a relatively higher carbon content, for example 12% Cr, 2%C, it may be thermally hardened. Because such a blade may have a surfacelayer which gives hardness values which are inconsistent with thehardness of the main body of the blade, the blade is treated to remove asurface layer of, for example, 0.001"-0.002" thick. Removal of such asurface layer will remove any surface layer having a hardness which isgreater than or less than the hardness of the main body of the blade dueto, for example, a carburised or decarburised surface layer. The layeris removed in conventional manner by mechanical abrasion with, forexample, grinding wheels and/or belt grinders.

Moreover, if the blade repair requires removal of blade material such asthe removal of the material in the region 14 referred to above, or ifmaterial has to be ground away in the region of the crack, as shown at112 above, so as to expose the interior of the blade, this removal isthen performed.

Thereafter, a series of hardness tests are carried out over the bladeincluding, if present, any regions exposed by removal of metal asmentioned above, so as to enable the hardness of the interior of theblade, as well as the exterior, to be determined.

The resultant hardness distribution pattern is then examined and anynecessary heat treatment which is required prior to repair can bedetermined, for example to stress relieve any such thermally hardenedblade.

The blade is then heat treated to stress relieve the blade andsubsequently the blade is repaired and stress relieved again, ifnecessary. The or each stress relieving or other heating operationnecessary on the blade is performed as hereinafter to be described.

Referring now to FIG. 4 there is shown an end part of the turbine bladeof FIG. 2 fitted with heating mats whereby a heating operation can beperformed. In the illustrated example the mats are for pre-heating butessentially the same features as hereinafter to be described apply toany heating operation, although the details of mat size, capacity,position and the electrical energy input, may vary depending upon theparticular heating operation to be performed, as well as blade size.FIG. 5 shows an end part of the blade of FIG. 3 similarly fitted withheating mats for the heating operation necessary for that mode ofrepair.

Referring now again to FIG. 4, the area of the blade surrounding thepart to be repaired is covered or surrounded with heating mats 30, 31,32, 33, 34 and 35. The heating mats may be of any suitable form and maycomprise, as shown in the heating mat 30, a continuous conductor 36passing through throughbores in a large number of ceramic beads 37.

Each of the heating mats 30 to 35 is controlled by a heating controlcircuit 40. The control circuit 40 has an input 41 to supply theelectrical energy to all the heating elements such as the heatingelement 36 of the mat 30 and a plurality of outputs 42, 43, 44, 45, 46and 47 so that the energy supplied to each of the heating mats 40 to 35may be independently controlled. The heating control circuit 40 has aninput 48 from master control unit 50. The master control unit 50 has apower supply 51 and is provided with inputs 52 to 56 from temperaturesensing means 57 to 61 respectively.

Further temperature sensing means 62 and 63 may be provided whichprovide input signals to the master control circuit 50 at inputs 64 and65 to alter the energisation of heating mats 30 to 35 should the actualtemperature of the turbine blade sensed by sensors 62 and 63 immediatelyadjacent the area to be repaired either be above or below an expectedvalue due to, for example, unexpected changes in ambient condition orthe length of time taken to carry out the brazing or welding operationwhich may increase or decrease the expected temperature.

The temperature sensing means 57-61 may comprise thermocouples which maybe welded to the blade.

When a set of blades for repair are presented for heating according to apredetermined heating regime (which may be for stress relief beforeand/or after repair, or preheating of the blade prior to repair, orheating during repair) a sample blade has applied thereto heating matsof a capacity, size and distribution which is expected to provide adesired heating distribution. If desired, two or more mats may besuperimposed at any position on the blade where heat input requiresthis. The temperature sensing means 57-63 are connected to the mastercontrol unit 50 and at least one monitoring means 57a-63a is providedassociated with each mat and preferably a plurality of monitoring meansare provided for at least one of the mats such as the monitoring means57a-57l shown associated with the mat 31. An electrical current ofsuitably adjusted current and voltage is then supplied to the mats bythe unit 40 under the control of the control unit 50 and the resultanttemperature distribution is determined by the monitoring means 57a-l,58a-63a, the output of all of which can be recorded if desired, forexample, using a chart recorder. When the heat treatment is to causemetallurgical change, for example if it is to be a stress relief heattreatment, the heat applied at this stage is less than that which willcause metallurgical change, for example 200°-300° C. The capacity, size,and distribution of the mats, as well as the electrical energy input,are adjusted as necessary empirically until the desired temperaturedistribution is achieved. Then the temperature is increased to thatrequired to effect the necessary heat treatment then the bladetemperature distribution is again monitored to ensure that the desiredtemperature distribution. is achieved. The relevant parameters of theheating means are noted as a heating specification.

The master control unit 50 can be programmed either manually or besupplied with recorded information from past analysis to control thesignal supplied to input 48 of heating control circuit 40 so as tocontrol each of the heating mats 30 to 35 with the amount of energyrequired to produce the desired temperature in the turbine blade 10.

Since ambient conditions may alter the actual temperature of the bladefrom the expected temperature given a predetermined amount of heat,sensors 57 to 61 can alter the signal produced by master unit 50 toheating control 40 so as to bring the temperature into line by alteringthe energisation of heating mats 30 to 35 to bring the temperature toits desired value.

After such heat treatment, if a stress relief or other heat treatmentaffecting the metallurgical properties of the material, the hardness ofthe blade may be again checked.

The relevant parameters of the heating means are noted as a heatingspecification.

Thereafter, all the blades, or at least a plurality of the bladespresented for repair, are then heat treated, preferably at the sametime, by replicating the heating specification that is by positioningsimilar mats in the same way and using the same electrical input to themats as that used for the heat treatment of the sample blade. However, aplurality of monitoring means, such as the sensors 57a-l. are notnecessary for each blade since it has been determined for the sampleblade that the appropriate temperature distribution is achieved with theparticular mat arrangement and energy input so long as the heatingspecification is replicated for all the blades.

However, if any of the heat sensors 57-63 or their associated circuitrywere to fail, then incorrect input signals would be given to the mastercontrol circuit 50 so that the energisation of the heating mats 30-35would be incorrectly controlled and accordingly a predetermined desiredtemperature distribution would not be achieved. This could lead toserious failure of the blade.

In order to overcome this problem, in accordance with the invention, amonitoring means is provided for each of the heat sensors 57-63.

The monitoring means comprises a single temperature sensitive device57a-63a separate from the temperature sensing means constituting theheat sensors 57-63. The temperature sensitive devices may each comprise,for example, a thermocouple which may be welded to the blade. Ifdesired, more than one such temperature sensitive device may be providedfor each heat sensor 57-63.

The thermocouples are positioned adjacent a respective temperaturesensing means 57-63 and are positioned in such proximity to thetemperature sensitive means that the repeatability of temperaturereadings can be ensured. Typically, one of the positions used for amonitoring means such as the monitoring means 57a-l, 58a-63a is used,since the temperature at such location in relation to the respectivetemperature sensing means 57-63 is known.

Because of variations in metal thickness due, for example, to change inblade section, the temperature of the blade at said location adjacentthe position of the temperature sensing means 57-63 may not be the sameas at the respective positions. The temperature sensed by the monitoringmeans 57a-63a may be higher or lower than the temperature sensed by theassociated temperature sensing means 57-63.

The monitoring means 57a-63a are connected to inputs 57b-63brespectively of a monitoring circuit 70.

The monitoring circuit 70 may be arranged to perform one or more of thefollowing functions.

a) provide a display, or recording of the temperature sensed by eachmonitoring means 57a-63a.

b) a display or recording of this temperature but modulated by apredetermined amount so as to be corrected so as to reflect thetemperature sensed by the associated temperature sensing means 57-63.

c) a thermal history by displaying, or recording, temperature againsttime. This may be done for a plurality of blades at the same time, e.g.on a multi-pen chart recorder.

d) an audible or visible warning, if the temperature sensed by any oneor more of the monitoring means varies by a predetermined amount fromthe temperature which should be sensed if the temperature at thetemperature sensing means is in the predetermined range.

e) an input signal to the master control unit 50 to disable the unit 50to interrupt the heating operation after, for example, a time delaywhich may be 20-30 seconds, or otherwise to control the master controlunit as desired.

f) utilise the monitoring means 57a-63a to control said application ofheat.

Alternatively, instead of using the monitoring circuit for this latterfunction, the monitoring means 57a-63a may be manually connected to themaster control unit 50 in place of an input 52-56 from a temperaturesensing means 57-61 which has failed.

The monitoring means 57a-63a are positioned adjacent to and in proximityto the temperature sensing means 57-63 at predetermined positions, whichpositions generally have been predetermined on the basis of empiricaltests, as described above, to ensure that the temperature sensed by themonitoring means accurately reflect either directly, or indirectly witha suitable adjustment factor, the temperature at the position of theassociated temperature sensing means.

Accordingly, the monitoring means enables the temperature as sensed byeach temperature sensing means to be checked so that failure of one ormore of the temperature sensing means does not lead to incorrect heattreatment of the blade.

Referring now to FIG. 5, turbine blade 110 shown in FIG. 3 is providedwith electrical heating elements 113, 114 which is supplied with currentby a control unit similar to the master unit 50 under the control ofsensing signals derived from temperature sensing means similar to thetemperature sensing means 57-63 and monitored by monitoring meanssimilar to the monitoring means 57a-63a described hereinbefore andconnected to a monitoring circuit similar to the circuit 70 describedhereinbefore.

After the blade has been heated for a sufficient length of time toachieve the desired temperature at different parts therealong (whereheating is required prior to repair), the building up the new shield 17to the turbine blade 10 by welding or other welding or brazing operationis performed. During this step in the repair which imparts a substantialthermal heat input to the blade, the heating mats 30 to 35 may beenergised or, since the heat capacity of the blade may be considerableenergisation may be terminated at this stage and the cooling processbegun.

The blade may now be subjected to further heat treatment from theheating mats 30 to 35 such as a stress relief operation. In some casesit may be desirable to maintain a maximum heated temperature of theblade for a certain period of time and then gradually decrease thetemperature of the blade over a predetermined time period.

A heating specification suitable for each heating operation isdetermined for a sample blade as necessary in a manner similar to thatdescribed above.

The hardness of a blade may be tested at each relevant stage, e.g.initially, after initial stress-relief, after repair and after furtherstress-relief, or after a desired one or combination of such steps.After final heat treatment the positions at which the temperaturesensing means 57-63 and monitoring means 57a-l, 58a-63a have been weldedto the blade are removed by mechanical abrasion. Moreover, if desired, a0.001"-0.002" layer may be removed over the whole blade surface prior tofinal hardness testing.

Although, generally a heating specification is determined empirically,where blades to be heated are the same as, or are similar to previouslyheated blades, a previously known heating specification may be used forall the blades, with or without performing a trial on a sample blade.Further, if desired, in a suitable case, a heating specification may becalculated rather than empirically derived and such a calculated heatingspecification may be used for all the blades, with or without performinga trial on a sample blade.

It should be appreciated that the details of the heating means and theirdistribution and the distribution of the heat sensing means describedhereinbefore are by way of example only, as are the modes of repair andthe present invention can be applied to any heating operation involvedin the repair of a turbine blade or like turbine component such as astator blade, nozzle, bucket or the like, all of which, in thestatements of invention hereof, are encompassed by the term "turbineblade".

The features disclosed in the foregoing description, or the followingclaims, or the accompanying drawings, expressed in their specific formsor in terms of a means for performing the disclosed function, or amethod or process for attaining the disclosed result, or a class orgroup of substances or compositions, as appropriate, may, separately orin any combination of such features, be utilised for realising theinvention in diverse forms thereof.

I claim:
 1. A method of repairing a turbine blade comprising performinga heating operation on the blade in which the blade is heated to achievea predetermined temperature distribution, sensing the temperature of theblade at least at one position by temperature sensing means andcontrolling the application of heat in accordance with the temperaturedetermined by said at least one sensing means and monitoring thetemperature of the blade at a location adjacent said at least oneposition with a monitoring means, and repairing said turbine blade.
 2. Amethod according to claim 1 wherein the method comprises the steps ofapplying heating means to selected areas of a turbine blade to berepaired, heating said areas of the turbine blade to desiredtemperatures, sensing the temperature of the blade at positionsassociated with said areas by respective temperature sensing means, andcontrolling the application of heat by said heating means in accordancewith the temperature determined by the respective temperature sensingmeans and monitoring the temperature of the blade at a location adjacenteach of said positions with said monitoring means.
 3. A method accordingto claim 1 wherein the monitoring is performed with a temperaturesensitive device separate from a temperature sensing means by which thetemperature at said at least one position is sensed.
 4. A methodaccording to claim 1 wherein the monitoring comprises the manualmonitoring of an indicating means.
 5. A method according to claim 4wherein the indicating means comprises at least one of an audible and avisible alarm signal.
 6. A method according to claim 4 wherein theindicating means is activated as a result of the temperature sensitivedevice sensing that the temperature of the blade at said location liesoutside said predetermined temperature distribution.
 7. A methodaccording to claim 1 wherein the monitoring means includes a visualdisplay unit which indicates the temperature sensed by the monitoringmeans and/or provides a historic record of temperature displayed againsttime.
 8. A method according to claim 1 wherein the monitoring meansprovides a control signal to a heating unit to disable or otherwisecontrol the heating unit if the temperature sensed by the monitoringmeans lies outside predetermined range.
 9. A method according to claim 1wherein the monitoring means may be substituted for the temperaturesensing means to control the application of heat.
 10. A method accordingto claim 1 wherein the monitoring means is disposed in a predeterminedrelationship to said at least one temperature sensing means and themonitoring means is responsive to a temperature sensed by the monitoringmeans which is related to the temperature sensed by the associatedsensing means in a predetermined manner.
 11. A method according to claim1 wherein the method of repair comprises welding or brazing cracks in ablade and/or re-building by welding or brazing and/or securing bywelding or brazing a hard metal shield at the outer end of the leadingedge of a turbine blade.
 12. A method according to claim 1 wherein themethod of repair comprises the steps of removing metal from the turbineblade in the area of a crack or other defect, replacing the removedmaterial with metal in a fused state by a welding or brazing operationand/or by securing an insert of solid metal to the blade by a welding orbrazing operation.
 13. A method according to claim 1 wherein the bladeis preheated in a controlled manner to a predetermined desiredtemperature before and/or during and/or after the repair operation. 14.A method according to claim 1 wherein, before or after the repairoperation, heat is applied to the turbine blade and the turbine blade ismaintained at a temperature, or a series of different temperatures, fora predetermined length of time such as to eliminate or substantiallyreduce, stress present in said blade.
 15. A method according to claim 1wherein the turbine blade is maintained at a predetermined temperatureduring the welding or brazing operation.
 16. A method according to claim1 including the step of removing a surface layer from at least part ofthe blade and then performing a hardness testing operation to determinea desired temperature distribution.
 17. A method according to claim 16wherein the blade has been thermally hardened prior to removing saidsurface layer.
 18. A method according to claim 1 wherein a plurality ofblades are repaired by first achieving a desired temperaturedistribution are for a sample blade by determining a heatingspecification by selecting, positioning and energizing at least oneheating means on the sample blade and then applying a similar heatingspecification to each other blade.
 19. A method according to claim 18when the sample blade is provided with a plurality of monitoring means,at spaced positions thereon, associated with a single temperaturesensing means and associated heating means, whilst each other blade isprovided with a lesser number, preferably only one, monitoring meansassociated with a single temperature sensing means and associatedheating means.
 20. A method according to claim 1 wherein electricalheating means are provided to heat the turbine blade to the requiredtemperature.